Top-Down Etching: Schematic representation of SHJ solar cell with localized front contacts and no full-area TCO – designed using a top-down etching process from TU Delft. (Source: Smits et al., Solar RRL; Delft University of Technology) 
Technology

Localized Front Contacts Boost HJT Solar Cell Efficiency To 23.4%

TU Delft researchers have developed a top-down fabrication method to create localized front contacts in rear-junction HJT solar cells

Rajeshwari Gattu

  • The localized contact HJT solar cell reached 23.4% efficiency, a 0.7% absolute gain over reference based on conventional architecture

  • Enhanced design increased short-circuit current density (Jsc) to 40.5 mA/cm² while keeping the fill factor high at 81%

  • Eliminating full-area TCO reduces indium use, with potential for improved long-term UV stability

A research team at Delft University of Technology has introduced a novel architecture for silicon heterojunction (HJT) solar cells, achieving an efficiency of 23.4% using a localized front contact design. The team says this approach reduces optical and electrical losses by eliminating full-area transparent conductive oxide (TCO) layers, while maintaining cell stability.

Traditionally, HJT cells use full-area transparent conductive oxide (TCO) layers for both lateral charge transport and anti-reflection, but this dual role often limits performance. The top-down method replaces the full-area TCO with localized contacts beneath the metal grid, reducing parasitic absorption while maintaining efficient charge collection.

Key to the method is a wet chemical etching technique using fuming HCl, allowing for precise removal of TCO without damaging adjacent layers. An 8-nm hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) ‘stopping layer’ was introduced to preserve passivation quality and support charge transport through the silicon bulk. Following TCO removal, the doped (n)-type silicon contact layers were also etched from the window regions using KOH, with the nc-SiOx:H stopping layer preventing damage to the passivating layer beneath.

After these selective etching steps, the window regions, which are now free of both TCO and doped layers, were coated with a double-layer anti-reflective coating (DLARC). This consisted of a SiNx:H layer, which provides surface passivation and a positive field effect, followed by a MgF₂ capping layer that enhances light in-coupling. This restored the anti-reflective and passivating functions in non-contacted regions while minimizing parasitic optical losses.

According to the research, the redesigned architecture improved the short-circuit current density (Jsc) by over 3 mA/cm² and reduced parasitic optical losses by 1.22 mA/cm². The final solar cell delivered a Jsc of 40.5 mA/cm², maintained a fill factor near 81%, and recorded a 0.7% absolute efficiency gain compared to reference based on traditional HJT designs.

In addition to performance gains, the method affects material use and stability. It reduces the need for indium, which is commonly used in TCO layers. It also allows UV-stable, non-conductive coatings in areas that do not require electrical contact. Simulations suggest that efficiencies above 28% are possible with further improvements in wafer and surface design.